Types of Facilities

• Above Ground

Detention basins

Dry detention ponds (a.k.a. dry ponds, extended detention basins, detention ponds, extended detention ponds) are basins whose outlets have been designed to detain stormwater runoff for some minimum time (e.g., 24 hours) to allow particles and associated pollutants to settle. Unlike wet ponds, these facilities do not have a large permanent pool of water. However, they are often designed with small pools at the inlet and outlet of the basin. They can also be used to provide flood control by including additional flood detention storage.
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Retention Basins (Wet Ponds)

Wet ponds (a.k.a. stormwater ponds, wet retention ponds, wet extended detention ponds) are constructed basins that have a permanent pool of water throughout the year (or at least throughout the wet season). Ponds treat incoming stormwater runoff by allowing particles to settle and algae to take up nutrients. The primary removal mechanism is settling as stormwater runoff resides in this pool, and pollutant uptake, particularly of nutrients, also occurs through biological activity in the pond. Traditionally, wet ponds have been widely used as stormwater best management practices.
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Bioretention

Bioretention areas, or rain gardens, are landscaping features adapted to provide on-site treatment of stormwater runoff. They are commonly located in parking lot islands or within small pockets of residential land uses. Surface runoff is directed into shallow, landscaped depressions. These depressions are designed to incorporate many of the pollutant removal mechanisms that operate in forested ecosystems. During storms, runoff ponds above the mulch and soil in the system. Runoff from larger storms is generally diverted past the facility to the storm drain system. The remaining runoff filters through the mulch and prepared soil mix. The filtered runoff can be collected in a perforated underdrain and returned to the storm drain system.
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Sand Filter

Sand filters are usually designed as two-chambered stormwater practices; the first is a settling chamber, and the second is a filter bed filled with sand or another filtering media. As stormwater flows into the first chamber, large particles settle out, and then finer particles and other pollutants are removed as stormwater flows through the filtering medium. There are several modifications of the basic sand filter design, including the surface sand filter, underground sand filter, perimeter sand filter, organic media filter, and Multi-Chamber Treatment Train. All of these filtering practices operate on the same basic principle. Modifications to the traditional surface sand filter were made primarily to fit sand filters into more challenging design sites (e.g., underground and perimeter filters) or to improve pollutant removal (e.g., organic media filter).
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Maintenance includes removal of trash and debris, removal of sediment from forebay, erosion repairs, inflow and outflow structure upkeep and repairs, care of sand bed, etc. Maintenance intervals vary depending upon vegetation growth and pollutant loadings, but should occur at least quarterly.

Infiltration Basin

An infiltration basin is a shallow impoundment which is designed to infiltrate stormwater into the soil. This practice is believed to have a high pollutant removal efficiency and can also help recharge the ground water, thus increasing baseflow to stream systems. Infiltration basins can be challenging to apply on many sites, however, because of soils requirements. In addition, some studies have shown relatively high failure rates compared with other management practices.
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Stormwater Wetland (Shallow Marsh)

Stormwater wetlands (a.k.a. constructed wetlands) are structural practices similar to wet ponds that incorporate wetland plants into the design. As stormwater runoff flows through the wetland, pollutant removal is achieved through settling and biological uptake within the practice. Wetlands are among the most effective stormwater practices in terms of pollutant removal and they also offer aesthetic and habitat value. Although natural wetlands can sometimes be used to treat stormwater runoff that has been properly pretreated, stormwater wetlands are fundamentally different from natural wetland systems. Stormwater wetlands are designed specifically for the purpose of treating stormwater runoff, and typically have less biodiversity than natural wetlands in terms of both plant and animal life. Several design variations of the stormwater wetland exist, each design differing in the relative amounts of shallow and deep water, and dry storage above the wetland.
A distinction should be made between using a constructed wetland for stormwater management and diverting stormwater into a natural wetland. The latter practice is not recommended because altering the hydrology of the existing wetland with additional stormwater can degrade the resource and result in plant die-off and the destruction of wildlife habitat. In all circumstances, natural wetlands should be protected from the adverse effects of development, including impacts from increased stormwater runoff. This is especially important because natural wetlands provide stormwater and flood control benefits on a regional scale.
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Maintenance includes management of vegetation including invasive species control, removal of trash, debris, and sediment, erosion repairs, inflow and outflow structure upkeep and repairs, etc. Maintenance intervals vary depending upon vegetation growth and pollutant loadings, but should occur at least quarterly.

Grassed Swales

In the context of BMPS to improve water quality, the term swale (a.k.a. grassed channel, dry swale, wet swale, biofilter, or bioswale) refers to a vegetated, open-channel management practices designed specifically to treat and attenuate stormwater runoff for a specified water quality volume. As stormwater runoff flows along these channels, it is treated through vegetation slowing the water to allow sedimentation, filtering through a subsoil matrix, and/or infiltration into the underlying soils. Variations of the grassed swale include the grassed channel, dry swale, and wet swale. The specific design features and methods of treatment differ in each of these designs, but all are improvements on the traditional drainage ditch. These designs incorporate modified geometry and other features for use of the swale as a treatment and conveyance practice.
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Maintenance includes management of vegetation, removal of trash, debris, and sediment (especially at check dams), and erosion repairs. Maintenance intervals vary depending upon vegetation growth and pollutant loadings, but should occur at least quarterly.

Permeable Paving

There are three types of permeable pavement systems: Interlocking block, pervious concrete paving, and porous asphalt.

Permeable interlocking concrete pavement (PICP) consists of manufactured concrete units that reduce stormwater runoff volume, rate, and pollutants. The impervious units are designed with small openings between permeable joints. The openings typically comprise 5% to 15% of the paver surface area and are filled with highly permeable, small-sized aggregates. The joints allow stormwater to enter a crushed stone aggregate bedding layer and base that supports the pavers while providing storage and runoff treatment. PICPs are highly attractive, durable, easily repaired, require low maintenance, and can withstand heavy vehicle loads. Figure 1 shows installed pavers in a Seattle, Washington residential neighborhood.
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Pervious concrete, also known as porous, gap-graded, or enhanced porosity concrete, is concrete with reduced sand or fines and allows water to drain through it. Pervious concrete over an aggregate storage bed will reduce stormwater runoff volume, rate, and pollutants. The reduced fines leave stable air pockets in the concrete and a total void space of between 15 and 35 percent, with an average of 20 percent. The void space allows stormwater to flow through the concrete as shown in Figure 1, and enter a crushed stone aggregate bedding layer and base that supports the concrete while providing storage and runoff treatment. When properly constructed, pervious concrete is durable, low maintenance, and has a low life cycle cost. Figure 2 shows a pervious concrete walkway installed at the EPA Headquarters in Washington, D.C.
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Porous asphalt, also known as pervious, permeable, "popcorn," or open-graded asphalt, is standard hot-mix asphalt with reduced sand or fines and allows water to drain through it. Porous asphalt over an aggregate storage bed will reduce stormwater runoff volume, rate, and pollutants. The reduced fines leave stable air pockets in the asphalt. The interconnected void space allows stormwater to flow through the asphalt as shown in Figure 1, and enter a crushed stone aggregate bedding layer and base that supports the asphalt while providing storage and runoff treatment. When properly constructed, porous asphalt is a durable and cost competitive alternative to conventional asphalt.
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Maintenance of permeable pavement systems involves the removal of sediments with sweeping/vacuuming systems. Power washing should be avoided. Maintenance intervals are based on sediment loads, but are typically 1 to 2 times per year. Modifications to other pavement maintenance protocol, such as snow plowing and seal coating are required.

On-Lot Treatment (Residential)

The term "on-lot treatment" refers to a range of practices designed to treat runoff from individual residential lots. The primary purpose of most on-lot practices is to manage runoff from rooftops and, to a lesser extent, driveways and sidewalks. Rooftop runoff, particularly from residential roofs, generally has low pollutant concentrations compared to other urban sources (Schueler, 1994b). Managing runoff from rooftops effectively disconnects these impervious surfaces, reducing a watershed's overall imperviousness. This is important because many of the deleterious effects of urbanization on water quality can be traced to fundamental changes in the hydrologic cycle caused by increases in impervious materials, like roofs, covering the landscape (Schueler, 1994a).

Although a variety of on-lot treatment options exist, all can be placed in one of three categories: 1) practices that infiltrate rooftop runoff; 2) practices that divert runoff to a pervious area; and 3) practices that store runoff for later use. The best option depends on the goals of a community, the feasibility at a specific site, and the preferences of the homeowner.

The practice most often used to infiltrate rooftop runoff is the drywell. In this design, the storm drain is directed to an underground rock-filled trench that is similar in design to an infiltration trench (see Infiltration Trench fact sheet). French drains or Dutch drains can also be used for this purpose. In these designs, the relatively deep dry well is replaced with a long trench equipped with a perforated pipe buried within the gravel bed to distribute flow throughout the length of the trench.

Runoff can be diverted to a pervious area or a treatment area using site grading, or channels and berms. Treatment options can include grassed swales, bioretention, or filter strips. The bioretention design can be simplified for an on-lot application by limiting the pre-treatment filter and, in some cases, eliminating the underdrain (see Bioretention (Rain Gardens) fact sheet). Alternatively, rooftop runoff can simply be diverted to pervious lawns, as opposed to flowing directly onto the street and then to the storm drain system.

Practices that store rooftop runoff, such as cisterns and rain barrels, are the simplest of all of the on-lot treatment systems. Some of these practices are available commercially and can be applied in a wide variety of site conditions. Cisterns and rain barrels can be particularly valuable in the arid southwest, where water is at a premium, rainfall is infrequent, and reuse for irrigation can save homeowners money.

Maintenance varies for each individual practice, but can usually be performed by the homeowner or grounds keepers in conjunction with landscape maintenance.

• Below Ground Facilities

Proprietary Separators

A variety of products called swirl separators or hydrodynamic structures have been widely applied to stormwater inlets in recent years. Swirl separators are modifications of traditional oil-grit separators. They contain an internal component that creates a swirling motion as stormwater flows through a cylindrical chamber. The concept behind these designs is that sediments settle out as stormwater moves in this swirling path, and additional compartments or chambers are sometimes present to trap oil and other floatables. There are several different types of proprietary separators, each incorporating slightly different design variations, such as off-line application.
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Proprietary Filters

Several underground vault systems that use proprietary filter cartridges are available. Each manufacturer has different cartridge designs. A variety of filter media is available to treat target pollutants based on research and testing. Sizing and design of these devices is based on the manufacturers information.

Maintenance of these facilities is based on manufacturers recommendations and engineering judgement. No maintenance should occur until a documented inspection determines it is required. Level I maintenance is an interim cleanout of sediment without replacing filter cartridges or filter media. Level II maintenance is a replacement of the cartridges or media in the cartridges with a cleanout of sediment from the vault. Sediment removal from underground facilities should be performed by qualified personnel with OSHA confined space entry training. Industrial vacuum equipment must be used. Removed material must be disposed of apprpriately.

Sand Filter

Underground Sand filters are usually designed as two-chambered stormwater practices; the first is a settling chamber, and the second is a filter bed filled with sand or another filtering media. As stormwater flows into the first chamber, large particles settle out, and then finer particles and other pollutants are removed as stormwater flows through the filtering medium. There are several modifications of the basic sand filter design, including the surface sand filter, underground sand filter, perimeter sand filter, organic media filter, and Multi-Chamber Treatment Train. All of these filtering practices operate on the same basic principle. Modifications to the traditional surface sand filter were made primarily to fit sand filters into more challenging design sites (e.g., underground and perimeter filters) or to improve pollutant removal (e.g., organic media filter).
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Maintenance of these facilities should occur only after a documented inspection that determines if maintenance is required. Level I maintenance is a cleanout of sediment from the forebay chamber of the structure and minor care of the surface of the sand bed, Level II maintenance includes a partial or complete removal and replacement of the sand filter bed and underdrain system if needed. Sediment removal from underground facilities should be performed by qualified personnel with OSHA confined space entry training. Industrial vacuum equipment must be used. Removed material must be disposed of appropriately.

Infiltration Trench

An infiltration trench (a.k.a. infiltration galley) is a rock-filled trench with no outlet that receives stormwater runoff. Stormwater runoff passes through some combination of pretreatment measures, such as a swale and detention basin, and into the trench. There, runoff is stored in the void space between the stones and infiltrates through the bottom and into the soil matrix. The primary pollutant removal mechanism of this practice is filtering through the soil.
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Maintenance of these facilities should occur only after a documented inspection that determines if maintenance is required. Level I maintenance is a cleanout of sediment from inflow points, pretreatment, and the surface of the stone, if accessible. Level II maintenance includes a partial or complete removal and replacement of the stone and filter cloth.

Catch Basin Inserts & Skimmers

Catch basins, also known as storm drain inlets and curb inlets, are inlets to the storm drain system. They typically include a grate or curb inlet, sometimes with a sump to capture sediment, debris, and pollutants. Catch basins inserts are often used in combined sewer overflow (CSO) watersheds to capture floatables and settle some solids, and they act as pretreatment for other treatment practices by capturing large sediments. Skimmers act in conjunction with sumps to removal large-particle sediments and floatables, including hydrocarbons, from the stormwater. The effectiveness of catch basins, their ability to remove sediments and other pollutants, depends on its design (e.g., the size of the sump) and on maintenance procedures to regularly remove accumulated sediments from its sump. Inserts designed to remove oil and grease, trash, debris, and sediment can improve the efficiency of catch basins. Some inserts are designed to drop directly into existing catch basins, while others may require retrofit construction.
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Maintenance of these facilities should occur only after a documented inspection that determines if maintenance is required. Level I maintenance is a cleanout of sediment, trash and debris from the insert or water surface if a skimmer is used. Level II maintenance includes a partial or complete removal and replacement of the insert unit or sediment cleanout from the inlet sump, in the case of a skimmer. Sediment removal from underground facilities should be performed by qualified personnel with OSHA confined space entry training. Industrial vacuum equipment must be used. Removed material must be disposed of appropriately.

Detention

Below ground detention systems store runoff in large diameter pipe systems, concrete vaults, gravel, or proprietary storage device. These structures are often very large because they temporary store flood flows and function like a surface basin. Hydraulic control is with a control structure that restricts flow with an orifice and a weir.

Maintenance of these facilities should occur only after a documented inspection that determines if maintenance is required. Level I maintenance is a simple cleanout of minor sediment, trash, and debris from the control structure. Inspection and maintenance of the control structure is very important to reduce the possibility of clogging, which can result in expensive sediment cleanout operations. Level II maintenance includes removal of trash, debris, and sediment from the entire storage area. Sediment removal from underground facilities should be performed by qualified personnel with OSHA confined space entry training. Industrial vacuum equipment must be used. Removed material must be disposed of appropriately.

In-line Facilities

In-line storage refers to a number of practices designed to use the storage within the storm drain system to detain flows. While these practices can reduce storm peak flows, they are unable to improve water quality and offer limited protection of downstream channels. Hence, EPA does not recommend using these practices in many circumstances. Storage is achieved by placing devices in the storm drain system to restrict the rate of flow. Devices can slow the rate of flow by backing up flow, as in the case of a dam or weir, or through the use of vortex valves, devices that reduce flow rates by creating a helical flow path in the structure.
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Maintenance of these facilities should occur only after a documented inspection that determines if maintenance is required. Level I maintenance is a simple cleanout of minor sediment, trash, and debris from the flow regulator. Inspection and maintenance of the flow regulator is very important to reduce the possibility of clogging, which can result in expensive sediment cleanout operations. Level II maintenance includes removal of trash, debris, and sediment from the entire storage area. Sediment removal from underground facilities should be performed by qualified personnel with OSHA confined space entry training. Industrial vacuum equipment must be used. Removed material must be disposed of appropriately.